feat(alerts): a Test alert button in the alert properties

Sits to the right of the Built-in video pill, labelled "Test alert". It enqueues one synthetic
Super Chat through the real AlertOverlayLayer, so what plays is exactly what a viewer triggers —
the custom clip when one is set, the built-in clip otherwise — and it goes through the same
output path (preview and encoder) as a live alert. Local only: nothing reaches YouTube and no
chat row is invented.

ViewModels/MainViewModel.Overlays.cs

@@ -119,6 +119,30 @@ public partial class MainViewModel : ViewModelBase
         set => SetProperty(ref _featureRequestEmail, value);
     }
 
+    /// <summary>
+    /// Properties-panel "Test alert" (creator ruling 2026-11-01): fire one synthetic Super Chat
+    /// through the REAL alert layer, so what plays is exactly what a viewer would trigger — the
+    /// custom clip if one is set, the built-in clip otherwise. Local only; nothing is sent to
+    /// YouTube and no chat row is invented.
+    /// </summary>
+    public RelayCommand TestAlertCommand { get; private set; } = null!;
+
+    private void TestAlert()
+    {
+        _alertLayer.Enqueue(new Models.ChatMessage
+        {
+            AuthorName = "Test viewer",
+            Message = "Test alert — this is what a Super Chat looks like.",
+            Timestamp = System.DateTime.Now,
+            IsSimulated = true,
+            IsSuperChat = true,
+            SuperChatAmount = 5.00,
+            SuperChatCurrency = "$",
+            SuperChatDisplayString = "$5.00",
+            Kind = Models.ChatEventKind.SuperChat,
+        });
+    }
+
     public ICommand OpenSettingsCommand { get; }
     public ICommand OpenBugCommand { get; }
     public ICommand OpenFeatureCommand { get; }